Upgrade `downgrade --help`: `downgrade` actually updates `pubspec.lock` (#2859)

diff --git a/lib/src/command/downgrade.dart b/lib/src/command/downgrade.dart
index 1a116b9..3e75775 100644
--- a/lib/src/command/downgrade.dart
+++ b/lib/src/command/downgrade.dart
@@ -14,8 +14,7 @@
   String get name => 'downgrade';
   @override
   String get description =>
-      "Downgrade the current package's dependencies to oldest versions.\n\n"
-      "This doesn't modify the lockfile, so it can be reset with \"pub get\".";
+      "Downgrade the current package's dependencies to oldest versions.\n\n";
   @override
   String get argumentsDescription => '[dependencies...]';
   @override